Thermal interface materials (TIMs) are critical to protect active semiconductor devices, such as microprocessors, from exceeding the operational temperature limit. They enable bonding of the heat generating device (e.g., a silicon semiconductor) to a heat sink or a heat spreader (e.g, copper and/or aluminum components) without creating an excessive thermal barrier. The TIM may also be used in the assembly of other components of the heat sink or the heat spreader stack that comprise the overall thermal impedance path.
Formation of a small thermal barrier is an important property of a TIM. The thermal barrier can be described in terms of effective thermal conductivity through the TIM and is preferably as high as possible. The effective thermal conductivity of the TIM is primarily due to the interfacial heat transfer coefficient as well as the (intrinsic) bulk thermal conductivity of the TIM. A variety of other properties are also important for a TIM depending on the particular application, for example: an ability to relax thermal expansion stresses when joining two materials (also referred to as xe2x80x9ccompliancexe2x80x9d), an ability to form a mechanically sound joint that is stable during thermal cycling, a lack of sensitivity to moisture and temperature changes, manufacturing feasibility, and cost.
Several classes of materials are being currently used as TIMs, for example, thermal greases, thermal gels, adhesives, elastomers, thermal pads, and phase change materials. Although the foregoing TIMs are adequate for many current semiconductor devices, the increased performance of semiconductor devices in the near future will render the presently known TIMs inadequate. Specifically, the thermal conductivity of current TIMs generally does not exceed about 5 W/mK and is typically less than about 1 W/mK. However, TIMs that form thermal interfaces with effective thermal conductivities of about 15 W/mK or greater will soon be needed.
One alternative to the foregoing TIMs is a solid metal sheet or preform made of indium or other low melting temperature alloys that act as the thermal interface layer. The metal TIMs ensure high thermal conductivity value (e.g., about 80 W/mK for an indium sheet). Metal TIMs may also exhibit a favorable solder or wetting behavior upon reflow which facilitates a low thermal interfacial resistance. During reflow, the solder and substrate are heated, the solder melts and wets by surface tension and/or local surface alloying. The interfaces consist of intermetallics or interdiffused metals with thermal properties that are frequently less desirable than those of the bulk TIM metal but much better than existing (polymer based) TIMs. In most cases, metallic TIMs have to be subjected to reflow in order to form reliable thermal interfaces. Metallic TIMs, however, can fail in certain applications due to the relatively large difference(s) between the coefficients of thermal expansion (CTEs) of the TIM and the semiconductor and/or heat sink components and the lack of compliance.
The present invention is directed to materials, processes and designs to improve the performance of a semiconductor to heat sink interface thereby increasing the flow of heat from an electronic (heat generating) device. Among the objects and features of the present invention, therefore, is the provision of a TIM which has a high thermal conductivity (e.g., greater than about 15 W/mK); the provision of a TIM which has a high interfacial heat transfer coefficient (i.e., greater than about 50 W/cm2xc2x0 C.); the provision of a TIM with a coefficient of thermal expansion (CTE) that is a compromise between semiconductor devices and heat sink components so that the TIM will resist separation from the semiconductor substrate or the heat sink components or resist damaging the semiconductor substrate; the provision of a TIM with improved resistance to thermal cycling induced stress failure; the provision of a TIM with improved resistance to moisture changes; the provision of a TIM with improved resistance to temperature changes; the provision of a TIM which bonds to a heat sink component and a semiconductor device at a temperature less than the failure temperature of an active (electronic) device; the provision of a TIM which enables bonding to a semiconductor substrate and a heat sink component without the use of a flux (i.e., the TIM preferably comprises a fluxless or active solder); and the provision of a TIM which is readily compatible with active semiconductor device manufacturing processes. In short, the provision of a TIM in which the bulk properties and the interface properties are optimal for heat transfer and reliability.
Briefly, therefore, the present invention is directed to a thermal interface material comprising a solder and a CTE modifying component.
The present invention is also directed to an active solder that wets at a temperature below about 300xc2x0 C. without extrinsic fluxing, the active solder comprises indium and an intrinsic oxygen getter selected from the group consisting of alkali metals, alkaline-earth metals, refractory metals, rare earth metals and zinc and mixtures and alloys thereof.
Further, the present invention is directed to an active solder that wets at a temperature below about 300xc2x0 C. without extrinsic fluxing, the solder comprises about 80% by weight gold, and about 20% by weight tin and an intrinsic oxygen getter selected from the group consisting of alkali metals, alkaline-earth metals, refractory metals, rare earth metals and zinc and mixtures and alloys thereof.
Additionally, the present invention is directed to an electronic device package comprising: a semiconductor substrate having a front surface and a back surface; an electronic device on the front surface of the semiconductor substrate; a heat sink component having a front surface and a back surface; and a thermal interface material bonding the back surface of the semiconductor substrate to the front surface of the heat sink component, the thermal interface material comprising a solder and a CTE modifying component that has a coefficient of thermal expansion that is less than about 10 xcexcm/mxc2x0 C.
The present invention is also directed to an electronic device package comprising: a semiconductor substrate having a front surface and a back surface; an electronic device on the front surface of the semiconductor substrate; a lid having a front surface and a back surface and a recess for receiving an insert; the insert being sized and shaped to fit within the recess in the lid, the insert having a front surface, a back surface, a surface in contact with the lid and a coefficient of thermal expansion that is between about that of the lid and about that of the semiconductor substrate; and a first thermal interface material bonding the back surface of the semiconductor substrate to at least a portion of the front surface of the insert.